1. In a large bowl, bring together the butter, light brown sugar and golden syrup with a spatula or an electric hand mixer until well combined. Add the egg and mix through until smooth, then add all of the dry ingredients, except for the demerara sugar, and fold through until you have a soft, squishy dough. Cover and place in the fridge for around 1 hour to firm up.
2. Preheat your oven to 190°C/Fan 170°C/ Gas 5 and line a baking sheet with greaseproof paper.
3. Weigh out 20g nuggets of the dough and roll between your hands to make neat balls. Roll each one in the demerara sugar to completely coat them. Pop them onto the baking sheet 10cm apart, to allow for spreading. Bake for 10 minutes, then leave to cool down completely.
4. Now make the icing. In a stand mixer fitted with the paddle attachment, or in a medium mixing bowl, beat together the butter and salt until pale and whippy. Add the cream cheese and beat together on a high speed for 1–2 minutes. Add the icing sugar in two stages, beating for a couple of minutes after each addition until you have a light, smooth, luxurious icing. Pop this into a piping bag fitted with a large round nozzle.
5. Pair up the cookies and turn half of them over, ready to fill. Pipe 5–6 small blobs of icing (about the size of a 50p piece) around the middle of the cookies, 1cm/½in from the edge. Place the other cookies on top and gently press down to allow the icing blobs to spread a bit and reach the edges of the sandwich. Place in the fridge until you’re ready to eat.